The objective of this research is to characterize the active and passive mechanical properties of mammalian myocardium and determine the nature and relative importance of the mechanisms which establish those properties. A newly-developed technique for the measurement of segment lengths in the central region of isolated papillary muscles will allow the limitations of previous methods to be overcome and will provide a new and more reliable approach to the study of mechanical function. This technique will be refined, validated by comparison with other techniques, and utilized to establish the static and dynamic characteristics of undamaged myocardial segments. The influence and interaction of mechanical and inotropic factors in the determination of these characteristics will be assessed. Such data will form the basis for estimation for the relative importance of these factors in the control of contraction and the likely mechanisms involved.